How Good Is Your #4 Starter?

When pundits talk about a free-agent pitcher, they often refer to him as a “#3 starter” or, say, a “#4 starter for a contender.” It’s common enough usage that most baseball fans know what that means, or at least know what the pundits are getting at.

Of course, this usage is extremely imprecise: one man’s #2 is another man’s #4, and there’s no clear way to settle the debate. Taken literally, a pitcher’s position in the rotation depends entirely on context: Zach Miner, the fifth-best starter on last year’s Tigers, had a lower ERA than any regular starter for the Royals.

My biggest beef with this kind of talk is that it invariably overestimates just how good pitchers should be. Ask most fans to list you some #1 starters and you might get 15 aces out of them; within the 30 top pitchers in baseball, there are some names that don’t seem to fit. Practically speaking, that means there aren’t enough #1 starters to go around. Ignoring for the time being that Luke Hudson was the best starter on his team, that top 30 list still includes names such as John Lackey, Chris Capuano, and Jason Jennings.

Further complicating the situation is the prevalence of pitcher injuries. Ben Sheets is one of those big-name aces, but while the Brewers planned for him to be their #1 guy last year, it was more frequently Capuano at the top of the rotation. The injury effect on the entire rotation is even more dramatic: many teams have to get thirty or more outings out of replacements. For instance, John Rheinecker, Carlos Marmol, and Joe Saunders each started 13 games last year.

The result of all of those injuries: the guy you sign as your #4 starter becomes your #3 guy, and your swingman enters the rotation for a month. An above-average rotation can look outstanding if everyone stays healthy: think of the 2006 Tigers (Jeremy Bonderman, Kenny Rogers, Nate Robertson, and Justin Verlander), or more dramatically, the 2003 Mariners, for whom five pitchers started all 162 games.

Shall We Crunch Some Numbers?

To get a better sense of what we mean (or, anyway, what we ought to mean) when we talk about rotation spots, let’s look at some stats. Ideally, we can come up with some numerical guidelines to easily eyeball where a pitcher fits into the average rotation.

For the purposes of this article, it’s necessary to define exactly what a #1 starter (or #2, or #3) is. To keep things as simple as possible, I used ERA as a measure of pitching ability. I also figured that each rotation spot accounts for 32 starts. On many teams, the #1 guy isn’t the same for the whole season. For example, let’s look at the 2006 Twins. Here are all of the pitchers who made more than one start for Minnesota last year:

By ERA, Francisco Liriano was the best of these guys, but he only made 16 starts. So, he made half of the “#1 starter” starts. Since Johan Santana is next in line, I assigned 16 of his starts to round out a composite #1 starter. Thus, the Twins #1 starter was half Santana, half Liriano. Santana’s remaining 18 starts were assigned to the composite #2 starter.

Intuitively speaking, that distribution is a reflection of the fact that, while Liriano was in the rotation, Santana was #2. When Liriano was in the bullpen or on the disabled list, Santana was #1. Here’s how that shakes out for the Twins staff:

Boof Bonser is a #2/#3 starter? Matt Garza a #4 guy fresh up from Triple-A? On the Twins staff last year, that was the case.

(A side note for those of you interested in methodology: I didn’t use play-by-play data for this analysis, so I don’t have each pitcher’s precise numbers as a starter. For the vast majority of pitchers, that’s not an issue, but for guys like Kyle Lohse, who made nine starts and 13 relief appearances, it gets tricky. For pitchers who made a start but made more appearances in relief, I estimated their starter ERA on the assumption that they averaged five innings per start and had an ERA exactly one run higher in the starting role. Using that technique, I estimate Lohse’s starter ERA at 7.47. Thus, in the complete table below, the Twins numbers are slightly different.)

The Results Are In

After going through that procedure for all thirty MLB teams, we can make some generalizations. To start with, here are the averages for each rotation position:

What immediately jumps out at me is how high the #4 and #5 ERAs are. If there’s one thing most people agree on when they talk about rotation spots, it’s that a guy with an ERA over 5.00 ought to be your #5 starter. As it turns out, fewer than half of major league teams could claim an ERA under 5.00 from their #4 spot.

In fact, only three teams in baseball got an ERA under 5.00 from their #5 spot: the Tigers (4.48), the White Sox (4.99), and the Padres (4.91). And if we adjusted for park, the Padres would sneak over 5.00. Only two other teams–the Giants (5.18) and the A’s (5.16) are under 5.50 from that position. Given the enormous difference between the best teams and the league averages, it’s all the more apparent just how valuable rotation depth can be.

To address the issue I raised at the outset, we can use these averages to come up with rough dividing lines between rotation spots. Armed with this data, you can take any pitcher’s ERA and eyeball where they would fit in to the average team’s starting corps. For instance, in the table below, if a pitcher is between 3.87 and 4.36, he is, on average, a #2 starter.

In other words, an AL pitcher who managed an ERA under 4.00 over 32 starts very likely qualifies as an ace. To take a few examples: Jason Schmidt is the “average” ace; a fringey #1 guy is Dontrelle Willis; an average #2 starter is Matt Cain, and the protoypical #4 is Luke Hudson. The rotation that was closest to major league norms was Milwaukee’s.

For those who are interested, here are last year’s complete results for all 30 major league teams: